docs: def get_children(self, usage_id_filter=None):
          """
          Return instantiated XBlocks for each of this blocks ``children``.
          """
          if not self.has_children:
              return []
  
          return [
              self.get_child(usage_id)
              for usage_id in self.children
              if usage_id_filter is None or usage_id_filter(usage_id)
          ]